RAPID COMMUNICATION Somadendritic Backpropagation of Action Potentials in Cortical Pyramidal Cells of the Awake Rat GYÖRGY BUZSÁKI AND ADAM KANDEL

Buzsáki, György and Adam Kandel. Somadendritic backpropagaThe rats were anesthetized with a mixture (4 ml/kg) of ketamine (25 mg/ml) , xylazine (1.3 mg/ml) , and acepromazine (0.25 mg/ tion of action potentials in cortical pyramidal cells of the awake rat. J. Neurophysiol. 79: 1587–1591, 1998. The invasion of fast (Na) ml) . For the simultaneous recording of field potentials and unit activity in different locations and cortical layers, silicon probes spikes from the soma into dendrites was studied in single pyramidal cells of the sensorimotor cortex by simultaneous extracellular rewith linear arrays of recording sites were used (Ylinen et al. 1995). The 16 recording sites of the silicon probe were 100 mm from each cordings of the somatic and dendritic action potentials in freely behaving rats. Field potentials and unit activity were monitored with other in the vertical plane (80 mm wide at the base, narrowing to 15 mm at the tip) . The probe was attached to a small movable multiple-site silicon probes along trajectories perpendicular to the cortical layers at spatial intervals of 100 mm. Dendritic action potenmicrodrive that was fixed to the skull of the rat and the probe was moved to the desired position during the experiment. Two stainless tials of individual layer V pyramidal neurons could be recorded up to 400 mm from the cell body. Action potentials were initiated at steel screw electrodes were driven into the bone covering the cerebellum and served as ground and indifferent electrodes. Thalamic the somatic recording site and traveled back to the apical dendrite at a velocity of 0.67 m/s. Current source density analysis of the stimulating electrodes (2 60-mm tungsten wires) were inserted in the n. ventralis posterolateralis. action potential revealed time shifted dipoles, supporting the view of active spike propagation in dendrites. The presented method is Physiological data were recorded on optical disks (1 Hz to 5 kHz) and sampled at 10 kHz/channel with 12-bit precision and suitable for exploring the conditions affecting the somadendritic propagation action of potentials in the behaving animal. analyzed off-line. Extracellular action potentials (‘‘spikes’’) and field activity rising from slower membrane potential changes were separated by 120 dB digital filters (0.5–5 kHz and 1–500 Hz, respectively) . The peaks of the largest amplitude extracellular I N T R O D U C T I O N spikes from a given site were detected by a peak searching software Recent in vitro experiments have provided evidence that (Ylinen et al. 1995) and the derived pulses served as a reference the dendritic membrane can exhibit electroresponsive propsignal for the construction of averaged unit waveforms from the erties and actively support propagation of action potentials wideband traces (1 Hz to 5 kHz). in dendritic compartments of cortical neurons (Johnston et The current source density (CSD) derivative of the simultaneal. 1996; Magee and Johnston 1995; Regehr et al. 1993; ously recorded extracellular voltage traces allowed for the continuSpruston et al. 1995; Stuart and Sakmann 1994). Active ous monitoring of the anatomical locations of sinks and sources of dendritic properties may be critical in synaptic integration action potentials. The results are presented as the unscaled second derivative of potential as a function of depth (Ylinen et al. 1995). and plasticity because calcium influx into the cell depends on A more detailed description of methods is available in Kandel and the number and frequency of fast (Na) spikes successfully Buzsáki (1997). The animals of the present report were part of invading the dendrites (Jaffe et al. 1992; Spruston et al. that study. 1995; Svoboda et al. 1997). Indeed, in vitro experiments suggest that dendritic fast action potentials may affect the efficacy of concurrently active synapses and influence synapR E S U L T S tic plasticity by enhancing Ca2 influx into the cell (Magee and Johnston 1997; Markram et al. 1997; Yuste and Denk Large amplitude action potentials, together with slow field 1995). Because a variety of conditions can affect the backpotentials, could be recorded from layers II to VI in the propagation and local boosting of the amplitude of dendritic awake rat (Fig. 1) . Thalamic evoked field responses and action potentials, it is reasonable to expect that local enspontaneously occurring spike-and-wave discharges resulted hancement of fast (Na) action potentials can be behaviorin spatially localized sinks and sources in the various cortical ally regulated (Buzsáki et al. 1996; Kamondi et al. 1997). layers (Castro-Alamancos and Connors 1996; Kandel and Monitoring dendritic activity of single cells in the behaving Buzsáki 1997). Layer IV was recognized by a prominent animal, however, was not possible until recently. Herein we sink during spike and wave patterns, sleep spindles, and report that dendritic action potentials of individual layer V thalamic-evoked responses (Kandel and Buzsáki 1997). Lopyramidal neurons in the sensorimotor cortex can be recal field events, such as extracellularly recorded action pocorded up to 400 mm from the cell body in the brain of tentials (spikes) were revealed by CSD analysis. Although awake rats. quantitative separation of all observable spikes was not at-